Conventionally, when an imaging device for taking color images is configured with a single-chip image sensor, photodiodes (pixels) are arranged like a squared grid or a 45° oblique squared grid, and color filters corresponding to color components such as R (red), G (green), and B (blue) are arranged on the photoreceptive surface of the respective photodiodes in a regular pattern such as “R G G B.” For the squared grid arrangement, an output image signal for the amount of one pixel is output from each pixel of the image sensor. For the 45° oblique squared grid arrangement, an output image signal for the amount of two pixels is output from each pixel of the image sensor. Thus, an output image is generated.
In such an imaging device for taking color images, in order to generate an accurate output image, the sampling frequency of the image sensor needs to be equal to or higher than double the spatial frequency of an object to be taken. That is, if the spatial frequency of the object is higher than the Nyquist frequency of the image sensor (half the sampling frequency), false colors (color moires) may occur at locations such as the boundary of a light portion and a dark portion of the object. Therefore, an optical low-pass filter is typically provided in the optical path with its cutoff frequency set to the Nyquist frequency of the image sensor, so that the imaging light directed to the image sensor is filtered.
As a conventional imaging device configured with a single-chip image sensor, an invention described in Japanese Patent Laid-Open No. 6-141330 has been proposed (Patent Document 1). The invention described in this specification mainly includes an image sensor of single- or two-chip type and a low-pass filter circuit having the cutoff frequency not higher than half the sampling frequency and detecting valid R, G, and B components from outputs of a video amplifier circuit.
Patent Document 1: Japanese Patent Laid-Open No. 6-141330